One hundred and sixty five survivors of isolated Medtronic-Hall aortic prosthesis operated on from September 1979 to September 1987 were studied. Ages varied from 14 to 68 years (m = 35.2) and 129 patients were male, 36 female. Preoperative diagnosis were 70 aortic insuficiency, 37 aortic stenosis and 39 double lesions. There were additionally 8 prosthetic dysfunctions, 8 acute infective endocarditis and 3 interventricular septal defects plus aortic insufficiency. One hundred and sixty three patients were followed (98.72), 9 of them being lost during the observation period. There were 45 late deaths, 59% SE 10.9% being the actuarial survival probality in 8 years. Twenty one patients suffered 26 embolic episodes, 69.8 SE 11.7% the probability of freedom from embolism and 39.7% SE 10.4% the chance of survival free from embolism. The rate of embolism episodes was 3,5% per patients/year in the entire series, 6 of them being lethal. In relation to the use of oral anticoagulation patients were divided into three sub-groups. Sub-group A included 144 patients, with a linearized incidence of 3.2% episodes per patients/year. Sub-group B included 21 patients who used anticoagulants after surgery, with an incidence of 1.9% per patients/year. Sub-group C comprised 9 patients who were put on anticoagulants after the occurence of an embolic episode. This sub-group presented 8.1 episodes per patients/year. It is concluded that it was not possible to doccument the influence of anticoagulation in the conditions prevailing during the observation of this series. After the occurence of one embolic episode the institution of oral anticoagulation was not effective in decreasing chance of its reccurence.